The identification of cellular genes whose expression or overexpression can cause cellular transformation and cancer is an important goal. The proposed researches, summarized below, are directed towards that objective. 1. A new method has been developed for cloning cellular mRNAs as cDNAs in E. coli or cultured animal cells. Since the procedure can generate nearly full length cDNA clones (the cDNA segments extend from the nucleotides adjacent to the poly A site up to, or a few nucleotides short of, the cap site) with high efficiency (about 10 5 cDNA recombinants per Mug of cell mRNA), the cloning vector has been constructed in such a way as to permit the expression of the cloned cDNA segment. Thus, it is feasible to attempt to clone cDNAs that could alter the phenotype of selected recipient cells or produce a product which permits the selection or identification of transduced cells. One objective is, therefore, to isolate cDNA copies of known and as yet unrecognized oncogenes by their ability to transduce normal to transformed cells. Similarly, cDNAs prepared from normal cells will be tested to determine if any can suppress the transformed cell phenotype of induced or natural cancer cells. 2. Studies of the structure and function of the SV40 virus early region promoter have shown that a 72 base pair tandem repeat sequence at about position -100 to -25 from the CAP-sites of the early mRNA, has a unique capacity to augment the transcribing activity of weak promoter sequences. The entire SV40 early promoter and its constituent parts (the promoter per se and the 72 bp tandem repeat) have been introduced into bacterial plasmids and will be tested for their ability to transduce normal to transfored cells by integrating at or near cellular genes whose overexpression lead to transformation. If transformed cells arise, the modified and corresponding normal genomic regions will be cloned and an analysis of their structure and oncogenic properties undertaken.